1. Field of the Invention
This invention relates to power converters (i.e., devices which convert one D.C. voltage or current to another D.C. voltage or current) and in particular to a power converter employed in a switching power supply and to components thereof.
2. Prior Art
Various push-pull proportional transistor base drive circuits have been successfully used for years in power converters. FIG. 1 illustrates a typical prior art proportional drive circuit for use in such a converter. In the structure of FIG. 1, the emitter current of Q3 causes a current to flow in the load winding (7-8) of T1 which by current transformer action applies a "load proportional" drive current to Q3 base. The termination of this drive condition occurs when Q5 of the regulator turns off and causes Q1 to saturate again. Q2 has remained saturated throughout the off period of Q1.
With Q1 and Q2 saturated, T1 is essentially shorted and Q3 turns off. Also the storage charge in C1 during the forward conduction of Q3 is now applied as an I.sub.B2 signal (i.e., a current out of the base of Q3 rather than into the base as shown) with a very low source impedance because T1 is shorted. As the I.sub.B2 pulse decays, Q3 turns off and its base remains reverse biased until I.sub.B1 is reapplied.
The circuit of FIG. 1 has certain problems in that R3 and R4 do not conduct enough current at low output currents to prevent magnetizing current in T1 from turning off transistors Q3 and Q4, respectively. Since the current in winding 7-8 (which is part of a regenerative feedback circuit) is low at light load, R3 and R4 would have to dissipate an undesirably large amount of power to eliminate this problem. An additional problem is that the circuit of FIG. 1 cannot be used for a full bridge converter of the type shown in FIG. 3 merely by adding another set of paralleled base drive windings such as windings P2 and P4 (FIG. 3). This is because the limited initial base drive will not turn on both parallel power transistors corresponding to Q1 and Q2 or Q3 and Q4 unless they are exactly matched. The transistor which turns on with the lower V.sub.BE will accept all the base charge and leave nothing for the other transistor. The circuit will neither start nor switch. This problem is further worsened if the full bridge stops with voltage across two transistors and no voltage across the other two. Thus the converter power switching stage shown in FIG. 3 may stop between turn-on of Q1 and Q2 on the one hand and Q3 and Q4 on the other hand with a different collector-to-emitter voltage across Q1 and Q4 as compared to the collector-to-emitter voltage across both Q2 and Q3. The transistor with the lower voltage (i.e., usually with no collector to emitter voltage across it) will hog the turn on base current and starve the other one, again resulting in no turn on and no switching.
Another problem with the cicuit of FIG. 1 is that transformer T1 isolates the primary P and secondary S circuits and often is designed to stringent product safety standards. In accordance with international safety standards, this requires insulation capable of withstanding 3750 VAC and results in large physical separation between primary and secondary windings. This results in high leakage reactance between primary and secondary windings and relatively tighter coupling between the primary (base) windings. During transistor turn off, ringing of the base waveforms turns on intermittently certain of the transistors Q1, Q2, Q3 and Q4 because the control (secondary) windings are poorly coupled. This results in a momentary short circuit across either Q1 and Q3 and/or Q2 and Q4, wasted power, and reliability problems.